Typically, liquid crystal displays (LCDs) utilize thin film transistors as switching elements. The liquid crystal display generally includes a thin film transistor substrate, on which the thin film transistors are formed.
FIG. 14 is a cross-sectional view of part of a conventional thin film transistor substrate. The TFT substrate 10 includes a base substrate 11, a gate electrode 12 formed on the base substrate 11, a gate insulating layer 13 covering the gate electrode 12 and the base substrate 11, an amorphous silicon (a-Si) layer 14 corresponding to the gate electrode 12 and formed on the gate insulating layer 13, two impurity-doped a-Si layers 18 formed on the a-Si layer 14 and being generally opposite to each other, a source electrode 15 formed on one of the impurity-doped a-Si layers 18, a drain electrode 16 formed on the other impurity-doped a-Si layer 18 and being generally opposite to the source electrode 15, and a passivation layer 17 covering the source electrode 15 and the drain electrode 16.
The gate electrode 12 is made of copper (Cu) to minimize resistance-capacitance (RC) delay caused by parasitic capacitors and parasitic resistors between the gate electrode 12 and a corresponding gate line (not shown). However, the adhesion strength between the copper gate electrode 12 and the base substrate 11 is weak. As a result, the gate electrode 12 may desquamate from the base substrate 11.
Referring also to FIG. 15, this is a cross-sectional view of part of another conventional thin film transistor substrate. The thin film transistor substrate 20 includes a base substrate 21, and a gate electrode 22 formed on the base substrate 21. The gate electrode 22 includes a barrier layer 223, a conductive layer 222, and a bonding layer 221, disposed in that order from top to bottom. The bonding layer 221 is formed on the base substrate 21 and is made of molybdenum (Mo). The conductive layer 222 is made of copper.
The main constituent of the base substrate 21 is silicon dioxide (SiO2). Most metallic materials have weak adhesion with silicon dioxide. Typically, the adhesion strength between a metallic material and silicon dioxide ranges from 0.5 joules per square meter (J/sq.m.) to 2 joules per square meter. That is, the effect using molybdenum as a bonding layer 221 to increase the adhesion strength between the gate electrode 22 and the base substrate 21 is limited. As a result, when the TFT substrate 20 sustains significant shock, the gate electrode 22 may desquamate from the base substrate 21.
What is needed, therefore, is a thin film transistor substrate and a method for fabricating the thin film transistor that can overcome the above-described deficiencies.